This invention relates to a process for the removal of heavy metals, calcium and sulphate from contaminated water, typically mine waters.
Effluent streams, and in particular acid mine drainage water, are generally acidic with pH values as low as 1.5. Another characteristic is the high levels of heavy metals, calcium and sulphate associated with the water. Prior to discharge into the environment these waste streams are normally neutralised with lime, a process which leaves large quantities of calcium sulphate in solution. The release of such waters into the environment poses a significant environmental challenge.
International patent application number PCT/GB98/01610 describes a process, generally referred to as “the SAVMIN process”, which was developed particularly for the treatment of sulphate-containing mine waters as well as sulphate-containing waste/effluent waters. This process allows for the effective removal of sulphate and calcium from effluent water with the use of amorphous aluminium trihydroxide followed by a subsequent recovery of the latter reagent by decomposing a waste product.
The SAVMIN process is fully described in the specification of the aforementioned patent application and the content of that specification is hereby incorporated fully into this specification.
In one stage of the SAVMIN process, a saturated calcium sulphate water stream (produced by preliminary steps) is combined with amorphous aluminium trihydroxide and a neutralising agent, for example hydrated lime, for the removal of sulphate and calcium from solution, to promote the precipitation of ettringite which is removed from the water stream, e.g. by settling, to produce a slurry.
This is followed by the recovery of amorphous aluminium trihydroxide by decomposing the ettringite slurry at a pH ranging from 4 to 8.5. The pH is lowered by adding sulphuric acid (H2SO4), resulting in the formation of a supersaturated calcium sulphate solution.
The solids resulting from the decomposition step are gypsum and amorphous aluminium trihydroxide. These solids are separated from one another by means of a suitable solid-solid separation unit, for example, but not limited to hydro-cyclone(s).
The recovered amorphous aluminium trihydroxide is recycled to treat a water stream containing sulphate and calcium. This recovery step ensures that the SAVMIN process is highly cost effective when compared to alternative processes such as ion exchange and membrane separation techniques.
An object of the present invention is to provide a mode of implementing the SAVMIN process in order to optimise the regeneration of amorphous aluminium trihydroxide and simultaneously to minimise the co-precipitation of basic aluminium sulphate. The latter aspect is important for it prevents a reintroduction of sulphate into the ettringite precipitation step during recycling of the precipitate amorphous aluminium trihydroxide.